Geofluids

Research Article

Rock Mass Classification for Columnar Jointed Basalt: A Case Study of Baihetan Hydropower Station

Table 7

Columnar jointed rock classification scheme.


Level	Sublevel	UCS (MPa)	Rock mass integrity	Weak plane tightness	Permeability (Lu)	Basic features

I		>200	Intact	Tight	<0.9	Rock mass is hard and intact with blocky structure. It is fresh without unloading effect and has extremely low permeability. Columnar joint is not developed and the rock mass in this level can be used as directly without any treatment.
II		100~200	Relatively intact	Tight	0.9~1.5	Rock mass is hard and relatively intact without or slightly weathered. Column is short and wide with blocky structure. The permeability is slight and rock mass can be used with local treatment.
III	III₁	50~100	Relatively intact~poor intact	Relatively tight~tight	1.5~2.6	Rock mass in this level is relatively intact and slightly weathered. Columnar joint develops and make the columns have columnar mosaic texture. The weak plane is tight and this level rock can be used under simple engineering treatment.
III	III₂	25~50	Poor intact	Moderate tight~relatively tight	2.6~3.7	Rock mass is poor intact and weak weathered. Columnar joints develop and make the columns have columnar mosaic texture. The weak plane is relatively tight and the permeability is poor. Rock mass can be used under certain treatment.
IV	IV₁	10~25	Relatively fractured~poor intact	Moderately tight~relatively loose	3.7~8.1	Rock mass is relatively fractured with weak unloading effect. Weak plane is moderately tight and develops in columns, making the rock in block-fracture structure. This kind of rock has relatively strong permeability and cannot be used as dam foundation directly.
IV	IV₂	3~10	Fractured~relatively fractured	Relatively loose	8.1~12	Rock mass is relatively fractured with intensive unloading effect. Weak plane develops adequately and the rock has cataclastic structure with strong water permeability. Rock mass in this level cannot be used and needs to be removed.
V		<3	Fractured	Relatively loose~loose	>12	There are large-scale intraformational faulted zones and open weak planes. The rock mass is extremely fractured with fragmental block structure.